This invention relates to an air tempering apparatus for use in an automotive vehicle having a duct through which air is directed into a passenger compartment.
Air tempering apparatus have sometimes utilized an air cooling heat exchanger for chilling air directed into a passenger compartment. The air cooling heat exchanger includes an evaporator having a variable refrigeration capacity. The evaporator is associated with a variable displacement compressor for supplying a refrigerant under pressure to the evaporator to adjust the refrigeration capacity of the evaporator. The compressor is controlled based upon ambient temperature and compartment temperature in such a manner as to have an increased displacement to increase the refrigeration capacity of the evaporator when the ambient temperature and/or the compartment temperature increases.
For example, U.S. Pat. application Ser. No. 388,444, filed Aug. 2, 1989, and entitled "Automotive Automatic Air Conditioning System with Variable Displacement Compressor" discloses an automotive air tempering apparatus operable in a low-temperature demist mode where the displacement or capacity of the compressor is controlled based upon a difference of an actual refrigerant temperature from a target refrigerant temperature changed at predetermined intervals of time between high and low values determined based upon the existing ambient temperature so as to dehumidity the passenger compartment to a sufficient degree even at low ambient temperatures.
The low-temperature demist mode continues until the ambient temperature decreases below a certain value. If the low-temperature demist mode continues for a long period of time, the evaporator would freeze. In addition, a refrigerant temperature sensor is provided at a position near the refrigerant inlet port of the evaporator to sense the actual refrigerant temperature. Thus, the refrigerant temperature sensor would be influenced by the warm air introduced into the duct from the passenger compartment to indicate an actual refrigerant temperature somewhat higher than the correct value. This results in a greater difference between the actual and target refrigerant temperatures, causing an unnecessary compressor displacement increase to freeze the evaporator.